A plasma display panel is a display apparatus which contains a plurality of discharge cells, and is constructed to display an image by applying a voltage across electrodes discharge cells thereby causing the desired discharge cell to emit light. A panel unit, which is the main part of the plasma display panel, is fabricated by bonding two glass base plates together in such a manner as to sandwich a plurality of discharge cells between them.
In a plasma display panel, each of the discharge cells which are caused to emit light for image formation generate heat and each thus constitutes a source of heat, which causes the temperature of the plasma display panel as a whole to rise. The heat generated in the discharge cells is transferred to the glass forming the base plates, but heat conduction in directions parallel to the panel face is difficult because of the properties of the glass base plate material.
In addition, the temperature of a discharge cell which has been activated for light emission rises markedly, while the temperature of a discharge cell which has not been activated does not rise as much. Because of this, the panel face temperature of the plasma display panel rises locally in the areas where an image is being generated. Moreover, a discharge cell activated in the white or lighter color spectra generate more heat than those activated in the black or darker color spectra. Thus, the temperature of the panel face differs locally depending on the colors generated in creating the image. These localized temperature differentials can accelerate thermal deterioration of affected discharge cells, unless measures are taken to ameliorate the differences. Additionally, when the nature of the image on the display changes, the location for localized heat generation changes with the image.
Further, since the temperature difference between activated and nonactivated discharge cells can be high, and the temperature difference between discharge cells generating white light and those generating darker colors also can be high, a stress is applied to the panel unit, causing the conventional plasma display panel to be prone to cracks and breakage.
When the voltage to be applied to the electrodes of discharge cells is increased, the brightness of the discharge cells increases but the amount of heat generation in such cells also increases. Thus, those cells having large voltages for activation become more susceptible to thermal deterioration and tend to exacerbate the breakage problem of the panel unit of the plasma display panel. LEDs present similar issues with respect to heat generation as do PDPs. Similar issues arise in display devices other than emissive display devices, such as LCDs, where hot spots can limit the effectiveness or life of the device.
The use of so-called “high orientation graphite film” as thermal interface materials for plasma display panels to fill the space between the back of the panel and a heat sinking unit and to even out local temperature differences is suggested by Morita, Ichiyanagi, Ikeda, Nishiki, Inoue, Komyoji and Kawashima in U.S. Pat. No. 5,831,374, however, no mention of the use or distinct advantages of flexible graphite sheets is made. In addition, U.S. Pat. No. 6,482,520 to Tzeng discloses the use of sheets of compressed particles of exfoliated graphite as heat spreaders (referred to in the patent as thermal interfaces) for a heat source such as an electronic component. Indeed, such materials are commercially available from Advanced Energy Technology Inc. of Lakewood, Ohio as its eGraf® SpreaderShield class of materials.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional such as thermal and electrical conductivity.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in heat spreading applications. Sheet material thus produced has excellent flexibility, good strength and a high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc.
The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon compression of the sheet material to increase orientation. In compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “C” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.
However, there is a concern in the electronics industries in general that the use of graphite-based materials can result in graphite particles flaking off, with the result that the flakes can mechanically (i.e., in the same manner as dust particles) interfere with equipment operation and function and, more significantly, that due to the conductive nature of graphite, graphite flakes can electrically interfere with operation of the emissive display device. Although it is believed that it has been shown that these concerns are misplaced, they still survive.
Also, the use of adhesives to attach a graphite heat spreader to an emissive display device can be disadvantageous at times. More specifically, in the event rework (i.e., removal and replacement of the heat spreader) is needed, the adhesive bond can be stronger than the structural integrity of the graphite sheet; in this situation, the graphite sheet cannot always be cleanly lifted off the panel without the use of scrapers or other like tools, which can be time consuming and potentially damaging to the graphite sheet, the panel or both.
Thus, what is desired is a light-weight and cost-effective thermal spreader for emissive display devices, especially one which is isolated to prevent flaking off of graphite particles and which can be effectively removed from the device when needed. The desired spreader should be capable of balancing the temperature differences over the area of the device contacted by the spreader to thereby reduce thermal stresses to which the panel would otherwise be exposed, and to be able to function to reduce hot spots even where the locations of hot spots are not fixed.